This invention pertains to catalytic oxidation and removal of nitrogen oxides (NOx) from exhaust gases derived from combustion of hydrocarbon fuels. It pertains particularly to a bi-functional oxidation catalyst and methods for making same for advantageous use in a treatment process for such oxidation of NOx contained in combustion exhaust gases, The process utilizes an initial catalytic oxidation step and can be followed by further oxidation by a chemical oxidant such as ozone (O3) for providing essentially complete NOx oxidation and removal from the combustion gases.
Conventional burning of fossil fuels such as coal, heavy oils and fuel gases to generate heat and energy results in formation of undesired concentrations of nitrogen compounds such as NOx contained in the resulting combustion exhaust and flue gas streams, and contributes to undesirable air pollution in the atmosphere. At least about 95% of such NOx in combustion gases is in the form of nitric oxide (NO). Because nitric oxide (NO) is relatively inactive chemically, its removal from combustion flue gas streams by scrubbing with suitable liquids is difficult and inefficient.
Some prior efforts for oxidation of NOx contained in combustion gases have utilized chemical oxidants such as ozone. For example, U.S. Pat. No. 4,011,298 to Fukui et al discloses oxidation and removal of sulfur oxide and nitrogen oxide (NO) from combustion gases by mixing with an ozone-containing gas. U.S. Pat. No. 4,024,219 to Takahaski et al discloses removing nitrogen oxides in waste gases by oxidizing with nitric acid in presence of a porous adsorbent agent such as silica gel or molecular sieves. U.S. Pat. No. 4,035,470 to Senjo et al discloses removal of sulfur oxides and/or nitrogen oxides from waste gases by adding chlorine dioxide or ozone and scrubbing with an aqueous solution. U.S. Pat. No. 4,351,811 to Matsuda et al discloses removing NO and NO2 contained in exhaust gases by contact with a metallic oxide catalyst together with ammonia. U.S. Pat. No. 4,564,510 to Bechthold et al discloses removing nitrogen oxides from waste gases by oxidizing with an agent such as ozone to form NO2, then adsorbing the NO2. U.S. Pat. No. 4,971,777 to Firnhaber et al discloses removing SOx and NOx from industrial furnace exhaust gases containing 0.5 vol % O2 by thermally oxidizing NO at 300-900 C followed by an adsorption step. Also, U.S. Pat. No. 5,206,002 to Skelly et al discloses treating combustion exhaust gases containing oxides of nitrogen and sulfur from an electric power plant by mixing with a chemical oxidant such as ozone (O3). U.S. Pat. No. 5,756,057 to Tsuchitani et al and U.S. Pat. No. 5,759,947 to Zhou disclose catalysts which can be used for removal of nitrogen oxides from combustion gases. However, it is apparent that known processes for oxidation and removal of nitrogen oxides from combustion exhaust or flue gases have generally been ineffective and undesirable, and that use of ozone alone for NOx oxidation involves undesirably high operating costs. Thus, further improvements in catalysts and processes for removal of NOx from combustion exhaust and flue gases are needed in industry for effectively reducing air pollution and smog and providing a clean air environment.
It is known that if the NOx contained in combustion flue gases could be efficiently oxidized to NO2 or N2O5, substantial amounts of such oxidized nitrogen compounds could then be effectively removed by liquid scrubbing of the treated flue gases. However, because oxidation of nitrogen oxide NO by O2 is very slow, improved methods for such oxidation are needed. Combustion flue gases containing NOx usually also contains some unreacted oxygen, such as 5-10% by volume. It is believed that such oxidation of NOx could be substantially enhanced by initial adsorption of the NO and the small concentration of O2 contained in combustion flue gases on a suitable catalytic surface, which also provides simultaneous desorption of an oxidized form of NOx such as NO2 from the catalytic surface. Such catalytic oxidation step for NOx could be followed by further chemical oxidation such as with ozone (O3) to form N2 O5. Because the oxidized NOx has much greater solubility in suitable scrubbing liquids, substantially all of the NOx contained in combustion exhaust or flue gases could be effectively removed in the form of oxyacids or salts.